Process for rendering an ash inert, artificial pozzolana obtained by means of the said process

ABSTRACT

Process for rendering inert an ash originating from the incineration of municipal waste, according to which the ash is subjected successively to treatment ( 19 ) with a water-soluble phosphate ( 20 ) in the presence of water, under conditions adjusted to crystallize hydroxyapatite and/or whitlockite, and to calcination ( 22 ). Artificial pozzolana, obtained by subjecting an ash originating from the incineration of municipal waste to such an inerting process.

The invention relates to a process for rendering inert ash originatingfrom the incineration of municipal waste.

The incinerators used to destroy municipal waste (domestic waste andhospital waste) produce generally large volumes of ash, both in the formof clinker in the incineration furnaces and of fly ash entrained in theflue gases. The mineralogical composition of this ash does not vary byvery much, whatever its origin, and the following are generallyencountered therein, although in proportions which can vary by as muchas 100%, indeed even more; alkali metal chlorides (NaCl and KCl),calcium (usually in the form of calcium carbonate, calcium sulphate,calcium hydroxide or calcium hydroxychloride, in particular calciumhydroxide and calcium sulphate), quartz, vitrified aluminosilicates,heavy metals in the metallic or combined form (in particular tin, zinc,lead, cadmium, mercury/copper and chromium), chlorinated organicderivatives and nonincinerated materials. The nonincinerated materialsusually include aluminium metal.

International Application WO 00/29095 discloses a process for renderinginert waste originating from incinerator flue gases, according to whichthis waste is dispersed with a hydraulic binder in water and thenfiltered off.

The process was improved by using, for the filtration stage, a beltfilter, which makes it possible to subject the filtration cake toefficient washing. This washing of the cake improves the mechanicalproperties and the resistance to leaching of the inerted waste resultingfrom the process.

European Patent EP-883 585 [Solvay (Société Anonyme)] provides a processfor rendering inert ash comprising heavy metals. According to this knownprocess, the ash is first washed and filtered, in order to remove thewater-soluble materials present therein, and the aqueous cake collectedfrom the filtration is treated with phosphoric acid or an alkali metalphosphate. The phosphate-comprising residue thus obtained issubsequently calcined and then a hydraulic binder and water are added toform a hydraulic mortar. On conclusion of this known process, a solidinorganic mass is obtained which is substantially inert with respect toatmospheric agents and which conforms to the standards for toxicity byleaching, in particular the TCLP (Toxicity Characteristic LeachingProcedure, USA) standard.

These known processes apply to all types of ash, in particular to flyash originating from the incineration of domestic or hospital waste.They involve a hydraulic binder.

In the case of ash originating from the incineration of municipal waste,an improvement to these known processes has now been found which makesit possible to dispense with the hydraulic mortar without harming theinert nature of the treated ash.

Consequently, the invention relates to a process for rendering inert anash originating from the incineration of municipal waste, according towhich the ash is subjected successively to treatment with awater-soluble phosphate in the presence of water and to calcination;according to the invention, the treatment with the phosphate is carriedout under conditions adjusted to crystallize hydroxyapatite and/orwhitlockite.

In the process according to the invention, the ash comprises a clinkerfrom a furnace for the incineration of municipal waste and/or a fly ashwhich was separated from the flue gas originating from such anincineration furnace.

The term “municipal waste” is intended to denote domestic waste andhospital waste. This waste normally comprises metals (including heavymetals and aluminium), calcium-comprising compounds (generally calciumhydroxide and calcium sulphate), sodium-comprising compounds (inparticular sodium chloride) and organic compounds (in particularchlorinated organic compounds and items made of plastic, especially madeof poly(vinyl chloride)).

The term “heavy metals” is intended to denote metals with a density atleast equal to 5 g/cm³, and beryllium, arsenic, selenium and antimony,in accordance with the generally accepted definition (Heavy Metals inWastewater and Sludge Treatment Processes, Vol. I, CRC Press Inc., 1987,page 2).

The amount of heavy metals in the ash subjected to the process accordingto the invention depends on the origin of the municipal waste. It isusually between 0.5 and 15 parts by weight per 100 parts by weight ofdry matter of the ash, more generally between 3.0 and 10 parts by weightper 100 parts by weight of the ash.

Aluminium is generally present in the ash in an amount of 2.0 to 8.0parts by weight per 100 parts by weight of the ash, more generally in anamount of 3.0 to 5.0 parts by weight per 100 parts by weight of the ash.

In the process according to the invention, the water-soluble phosphatereacts with the calcium of the ash, forming calcium phosphate. Thewater-soluble phosphate is generally phosphoric acid or an alkali metalphosphate, for example sodium phosphate. Orthophosphoric acid ispreferred.

In accordance with the invention, the treatment with the phosphate iscarried out under conditions adjusted in order for the calcium phosphatewhich is formed to be in the form of crystals of hydroxyapatite and/orof whitlockite.

Hydroxyapatite is a mineral of general formula Ca₅(PO₄)₃(OH).Whitlockite is a mineral of general formula Ca₉Fe_(x)Mg_(1-x)H(PO₄)₇,where x is an integer or fractional number between 0 and 1.

The crystals of hydroxyapatite and of whitlockite have the property ofallowing, by isomorphism, substitution of a portion of their calciumatoms by atoms of heavy metals. The amount of hydroxyapatite and/or ofwhitlockite crystallized must consequently be sufficient for theseminerals to absorb the heavy metals of the ash in their respectivecrystal latices. The optimum amount of hydroxyapatite and/or ofwhitlockite to be crystallized will consequently depend on the amount ofheavy metals present in the ash and it must consequently be determinedin each specific case by a routine operation in the laboratory. Theamount of hydroxyapatite and/or of whitlockite to be crystallized willitself condition the amount of water-soluble phosphate which has to beadded to the ash. Depending on the origin of the latter, the amount byweight of water-soluble phosphate employed (expressed in the H₃PO₄ form)can vary, for example, from 10 to 20% of the weight of dry matter of theash. Furthermore, the formation of hydroxyapatite and of whitlockiterequires the presence of calcium in the ash. Depending on the amount ofhydroxyapatite and/or whitlockite to be crystallized, the amount byweight of calcium (expressed in the CaO form) can vary, for example,from 10 to 35% of the weight of dry matter of the ash. In an alternativeform, if required, additional calcium (in the metallic or combined form)has to be added to the ash before or during the addition of thewater-soluble phosphate in order to obtain the required amount ofcrystals of hydroxyapatite and/or of whitlockite. Use is advantageouslymade of calcium carbonate, preferably avoiding basic calcium compounds,in particular calcium hydroxide.

In the process according to the invention, the water-soluble phosphateis added to the ash in the presence of water. The water must be presentin an amount at least sufficient to crystallize the hydroxyapatiteand/or the whitlockite. In practice, it is present in an amount ofgreater than 10% (preferably at least equal to 25%) of the weight of drymatter of the ash. In practice, it is not advantageous for the amount ofwater employed to exceed 100% of the dry matter of the ash. The valuesfrom 30 to 75% are generally highly suitable.

In the process according to the invention, the calcination has a twofoldfunction. First, it serves to destroy the organic compounds of the ash.Secondly, it brings about a recrystallization of the hydroxyapatiteand/or of the whitlockite. The calcination is normally carried out at atemperature of greater than 400° C., preferably at least equal to 600°C. It is not advantageous to exceed a temperature of 1000° C. Thetemperatures from 600 to 950° C. are especially advantageous. Thecalcination can be carried out in an inert atmosphere (for example,under a nitrogen atmosphere). It is preferable to carry out thecalcination in the presence of air, so as to bring about the combustionof the organic compounds, in particular halogenated organic compounds.

The recrystallization of the hydroxyapatite and/or of the whitlockitehas the advantageous result of strengthening the insoluble nature ofthese compounds in water.

In an advantageous embodiment of the process according to the invention,before the treatment of the ash with the phosphate, the ash is subjectedto alkaline aqueous washing at a pH of greater than 8.5, for examplefrom 9 to 14, preferably from 9.5 to 13. In this embodiment, the pH ismeasured on the aqueous solution collected from the washing. Thisembodiment of the process is targeted at removing the water-solublecompounds of the ash. It exhibits the additional advantage that the ashcollected from the washing is found under the optimum conditions forobtaining the crystallization of hydroxyapatite and/or of whitlockiteduring the treatment with the water-soluble phosphate.

In another specific embodiment of the process according to theinvention, which is especially suited to the case of an ash comprisingchromium in the hexavalent state, a reducing agent is added to the ashcollected from the treatment with the phosphate. In this embodiment ofthe process, the function of the reducing agent is to reduce thehexavalent chromium in order to bring it to a lower valency state. Ironmetal (for example iron filings) and carbon (for example active carbon)constitute preferred reducing agents. The reducing agent isadvantageously employed in an amount by weight substantially of between0.3 and 1% by weight of the ash.

The ash collected from the process according to the invention exists inthe form of a pulverulent or granular inorganic mass which is inert withrespect to the environment and atmospheric agents and which conforms tothe standards for toxicity by leaching, particularly the TLCP standarddefined above. This inorganic mass exhibits the noteworthy andunexpected property of having a pozzolanic capability which marks it outfor forming hydraulic binders.

In the case where the ash subjected to the process according to theinvention comprises calcium sulphate, it may prove to be desirable todecompose the latter. This is because it has been observed that if, onconclusion of the calcination, the ash comprises calcium sulphate, thelatter has a detrimental effect on the hydraulic properties of the ash.

To this end, in a specific embodiment of the process according to theinvention, the ash is subjected, before or after the calcination, towashing with a solution of sodium carbonate or of sodium hydroxide (forexample, at pH 13), so as to dissolve the aluminium by oxidation of thelatter. The washing can be carried out at ambient temperature or at ahigher temperature, for example from 40 to 75° C.

In an especially advantageous embodiment of the process according to theinvention, the ash is washed with an aqueous alkali metal carbonatesolution at a pH of greater than 10 to decompose the calcium sulphate.The washing is advantageously carried out at a pH of 12 to 13. In thisembodiment of the invention, the pH is measured on the aqueous solutioncollected from the washing. In the embodiment which has just beendescribed, the washing of the ash with the sodium carbonate solution canbe carried out on the ash downstream of the calcination. However, it ispreferable, according to an advantageous alternative form of theinvention, for the washing with the aqueous sodium carbonate solution tobe carried out at the same time as the washing of the ash, upstream ofthe treatment with the water-soluble phosphate.

It has been observed that in the case where the ash recovered from thecalcination comprising aluminium metal, the latter has a detrimentaleffect on the pozzolanic properties of the ash, resulting inuncontrollable swelling of the mortars.

To this end, in a preferred embodiment of the process according to theinvention, the ash is subjected to oxidizing heating in order to oxidizethe aluminium metal to alumina. For the implementation of thisembodiment of the invention, the oxidizing heating is advantageouslycarried out at the same time as the calcination, the latter then beingcarried out in an oxidizing atmosphere (for example, air) at atemperature of greater than 800° C., preferably of 900 to 1000° C.

The ash collected on conclusion of the process according to theinvention can be stored in a landfill site without risk to theenvironment (in particular to ground water and surface water). It canalso be recovered in value, such as in civil engineering works, forexample in road metals or as filling material in bitumen road surfaces.Because of its pozzolanic properties, the ash collected on conclusion ofthe process according to the invention finds an especially advantageoususe in the manufacture of hydraulic cements.

Consequently, the invention also relates to an artificial pozzolana,obtained by subjecting an ash originating from the incineration ofmunicipal waste to an inerting process in accordance with the invention,and to hydraulic binders comprising this artificial pozzolana.

The invention is illustrated by the following description of the singleFIGURE of the appended drawing, which represents the diagram of a plantemploying a specific embodiment of the process according to theinvention.

The plant represented in the FIGURE comprises a furnace 1 in whichmunicipal waste 2 is incinerated. A clinker 3, on the one hand, and aflue gas 4, on the other hand, are collected from the furnace 1. Theflue gas 4 is laden with fly ash and in addition is contaminated bytoxic gaseous compounds, particularly hydrogen chloride and volatileheavy metals. It is first treated in a dust separator 5 (for example, anelectrostatic precipitator), where the fly ash 6 is separated therefrom.The dedusted flue gas 7 is subsequently treated in a purification device8, known per se, in order to extract therefrom the acidic gaseouscompounds, and then it is discharged to the chimney 9.

The clinker 3 and the fly ash 6 are mainly composed of inorganiccompounds and of chlorinated organic compounds, in particular dioxinsand furans.

The clinker is treated in a mill 10 and the milled clinker 11 issubsequently introduced with the fly ash 6 into a mixing chamber 23. Anash 24 is collected from the chamber 23 and is sent to a washing chamber12. The ash 24 is dispersed, in the washing chamber 12, in an amount ofwater 13 sufficient to dissolve substantially all the water-solublecompounds present in the ash. In addition, sodium carbonate 14, intendedto react with the calcium sulphate of the ash to form insoluble calciumcarbonate and sodium sulphate, which passes into solution, is introducedinto the chamber 12. The pH in the washing medium of the chamber 12settles at approximately 12.5. An aqueous-suspension 15 is collectedfrom the washing chamber and is treated immediately on a filter 16. Anaqueous solution 17, on the one hand, and an aqueous cake 18, on theother hand, are separated from the filter 16.

The cake 18 comprises most of the heavy metals, in addition to thedioxins and the furans, of the ash 24. In addition, it comprises calcium(mainly in the form of calcium hydroxide and calcium carbonate) and isimpregnated with water In accordance with the invention, it isintroduced into a reaction chamber 19, where orthophosphoric acid 20,extra water 25 and iron filings 26 are added thereto. The phosphoricacid 20 reacts with the calcium compounds and with the water tocrystallize hydroxyapatite and/or whitlockite. The function of the ironfilings 26 is to reduce the hexavalent chromium. An ash 21 comprisingcrystals of hydroxyapatite and/or of whitlockite is collected from thereaction chamber 19. It is transferred to a calcination chamber 22,where it is heated at a temperature of approximately 950° C. in thepresence of air for a time sufficient to decompose the dioxins andfurans, to oxidize the aluminium metal to the form of alumina and torecrystallize the hydroxyapatite and/or the whitlockite. A drypulverulent mass 27 which is inert and which exhibits the properties ofan artificial pozzolana is collected from the calcination chamber 22.The artificial pozzolana 27 can advantageously be used in themanufacture of hydraulic binders.

The examples described below will reveal the advantage of the invention.

In these examples, a fly ash from the incineration of domestic waste wastreated, this fly ash having the composition which appears in Table 1below (analysis carried out by X-ray fluorescence).

TABLE 1 Constituent Content (mg/kg) Al  27 209 Ba  2 864 Br    913 Ca131 055 Cd    494 Cl 170 814 Cr    636 Cu  1 430 Fe  12 688 K  90 251 Mg 10 447 Mn    701 Mo    576 Na  11 894 Nb    10 Ni    101 P  3 246 Pb 11 226 Rb    121 S  50 698 Sb  1 020 Si  44 950 Sn  1 502 Sr    516 Ti 8 224 V    107 Zn  18 123

EXAMPLE 1

100 g of ash were withdrawn and were subjected to a washing operation.To this end, the ash was dispersed in an aqueous solution composed of 1l of demineralized water comprising 50 g of sodium carbonate and theaqueous suspension thus obtained was subjected to moderate stirring for2 hours at ambient temperature.

The aqueous suspension was subsequently filtered, the filtration cakewas dried and the dry cake was weighed: 78.2 g.

70 g of the dry cake were withdrawn and were subjected to the inertingprocess. To this end, 70 g of water and 7 g of phosphoric acid (whichcorresponds to 0.1 kg of phosphoric acid per kg of dry cake) weresuccessively added to it and the resulting mixture was calcined at 750°C. for 1 hour. The dry powder collected on conclusion of the calcinationwas subjected to a standardized test of toxicity by leaching. To carryout this test, 30 g of the powder were withdrawn and were subjected tothree successive leachings with 300 ml of demineralized water. The threeleaching solutions were collected and mixed and the composition of themixture of the leaching solutions which is thus obtained was analysed.The result of the analysis appears in Table 2 below.

TABLE 2 Contents Constituents (mg/kg of solution) Al 9.5 As 0.024 Ba 0.2Ca 5.5 Cd <0.01 Co <0.04 Cr 0.42 Cr (VI) <0.2 Cu 0.01 Fe <0.02 Hg 0.03Mg 0.3 Mn <0.01 Mo 0.84 Ni <0.03 Pb <0.3 Sb 0.17 Sn <0.02 Sr 0.2 Ti<0.01 Tl 0.3 V <0.03 Zn 0.05

EXAMPLE 2

The test of Example 1 is repeated under the following conditions:

Washing solution: 1 l of demineralized water, as such, without sodiumcarbonate, Phosphoric acid: 4.5 g (which corresponds to 0.1 kg/kg of drycake), Calcination: 750° C. for 1 hour.The results of the test of toxicity by leaching are recorded in Table 3below.

TABLE 3 Contents Constituents (mg/kg of solution) Al 16 As <0.01 Ba 0.2Ca 778 Cd <0.01 Co <0.03 Cr 2.4 Cr (VI) 0.65 Cu 0.02 Fe <0.01 Hg 0.005Mg 2.5 Mn <0.001 Mo 2.8 Ni <0.03 Pb <0.50 Sb 0.01 Sn <0.05 Sr 3.9 Ti<0.02 Tl <0.50 V <0.03 Zn

EXAMPLE 3

In order to demonstrate the pozzolanic nature of the treated ash, thefollowing comparative test was carried out. 75 g of road hydraulicbinder “ARC 3” (composed of 78% slag, 11% quicklime and 5% clinker) weremixed with 25 g of ash treated according to the invention. The resultingmixture was mixed with water and subjected to setting and curing. Themechanical properties of the mortar obtained were measured according tothe EN 196-1 standard and were compared with the properties of a mortarobtained starting from 100% of road hydraulic binder “ARC 3”.

Flexural Compressive Term strength strength (days) (MPa) (MPa) 100% Roadbinder “ARC 3” 60 d 7.4 32.0 75% Road binder “ARC 3” + 60 d 7.3 33.2 25%treated ashThe substitution of 25% of binder by treated ash does not reduce themechanical properties of the mortar, which shows the pozzolanic natureof the treated ash.

The preceding examples show the good ability of the process according tothe invention to render inert the heavy metals of fly ash.

Comparison of the results of Example 1 with those of Example 2 revealsin addition the advantage of incorporating sodium carbonate in the waterused to wash the fly ash, as regards rendering inert toxic metals, inparticular chromium (especially chromium(VI)), aluminium, molybdenum andstrontium.

1. Process for rendering inert an ash originating from the incinerationof municipal waste, according to which the ash is subjected successivelyto treatment with a water-soluble phosphate in the presence of water andto calcination, characterized in that the treatment with the phosphateis carried out under conditions adjusted to crystallize hydroxyapatiteand/or whitlockite, characterized in that a reducing agent is added tothe ash.
 2. Process according to claim 1, characterized in that, beforethe treatment with the phosphate, the ash is subjected to alkalinewashing at pH of greater than 8.5.
 3. Process according to claim 2,characterized in that the washing is carried out at a pH of 9.5 to 13.4. Process for rendering inert an ash originating from the incinerationof municipal waste, according to which the ash is subjected successivelyto treatment with a water-soluble phosphate in the presence of water andto calcination. characterized in that the treatment with the phosphateis carried out under conditions adjusted to crystallize hydroxyapatiteand/or whitlockite, characterized in that, in the case where the ashcomprised calcium sulphate, it is washed with an aqueous alkali metalcarbonate solution at a pH of greater than
 10. 5. Process accordingclaim 4, characterized in that the washing of the ash with the alkalimetal carbonate solution is carried out at a pH of 12 to
 13. 6. Processfor rendering inert an ash originating from the incineration ofmunicipal waste, according to which the ash is subjected successively totreatment with a water-soluble phosphate in the presence of water and tocalcination, characterized in that the treatment with the phosphate iscarried out under conditions adjusted to crystallize hydroxyapatiteand/or whitlockite, characterized in that in the case where the ashcomprises aluminum metal, it is subjected to a treatment for oxidationof the said aluminum metal, and the treatment of oxidation of thealuminum metal comprises an oxidizing heating of the ash.
 7. Processaccording to claim 6, characterized in that, to carry out the oxidizingheating of the ash, the calcination is carried out at a temperature of900 to 1000° C.